1. Field of the Invention
This invention relates to large crystal grains or a single crystal of molybdenum and a process for production thereof. More specifically, the invention relates to a large crystal grains or a single crystal of molybdenum containing calcium and/or magnesium and a process for production thereof.
2. Description of the Prior Art
Molybdenum has excellent strength at high temperatures and is expected to be useful as a heat resistant material. Heretofore, molybdenum element or its products have been supplied in the form of aggregates of small crystal grains, namely polycrystals. The serious defect of polycrystal molybdenum material is the brittleness of its grain boundary. In particular, its workability below room temperature is extremely poor. At high temperatures, its crystal grains become coarse to cause recrystallization brittleness, and its characteristic properties as a heat-resistant material cannot be fully exhibited. Hence, the molybdenum material has only limited applications. If molybdenum is available as large crystal grains or a single crystal being free from a grain boundary and having any desired large shape, it will not be mechanically broken because of the absence of grain boundary brittleness and can be used until it is worn out. This advantage will greatly enlarge the field of utilization of molybdenum, and it will find extensive use as a material for IC substrates, nuclear reactors, nuclear fusion reactors, heat generating members of usual furnaces, crucibles, electronic component parts, etc. Accordingly, large crystal grains or single crystals of molybdenum free from a grain boundary have been desired.
The article of N. K. Chen et al. entitled "Growth of Molybdenum Single Crystal" (Journal of Metals, June 1951, p. 461) describes a method of growing a single crystal of molybdenum by exerting a stress on pure molybdenum in the form of a wire and then heating it while providing a temperature gradient. But only a part of the wire is converted to a single crystal, and the length of the single crystal is only about 1.5 inches at the largest.
D. K. Bulychev et al. reported the effect of a working method on the recrystallization of molybdenum in the case of using a molybdenum rod ["Anomalous Grain Growth and Formation of Single Crystals on the Annealing of HydroExtruded Molybdenum", Fiz. metal. metalloved., 27 (1969), pp. 107-110]. The size of the resulting single crystal is 7 mm in diameter and 100 mm in length at the largest.
The article of D. J. Capp et al. entitled "The Preparation and Characterization of High-Purity Molybdenum Single Crystals" [Journal of the Less-Common Metals, 40 (1975), pp. 9-20]describes the preparation of high-purity molybdenum single crystals by a combination of electron-beam zone-refining and thermal anneals. They obtained high-purity molybdenum single crystals having a diameter of 10 mm and a length of 150 mm by this method. However, this method can give only a round rod-like product, and a high level of expert skill is required for its production.
The above-cited three references describe the production of single crystals of molybdenum, but fail to describe a method of easily obtaining large crystal grains or single crystals of molybdenum by adding specified amounts of additives.